A power device, a kind of semiconductor device, is used for a power conditioner of a photovoltaic power generation system and motor rotation control of home appliances or EVs (Electric Vehicles). Such power devices are one-packaged in an increasing number of products in order to reduce a footprint, improve performance with a shortened distance between semiconductor elements, or reduce a design load for users. One-packaging refers to housing and modularizing a plurality of power devices in a single package. A one-packaged product is called a power module.
A resin-molding power module is assembled by, for example, transfer molding.
For example, a semiconductor device according to transfer molding is disclosed in Patent Literature 1. FIG. 5 is a cross-sectional schematic diagram showing a conventional semiconductor device 100 relating to Patent Literature 1.
In the semiconductor device 100 of FIG. 5, two power semiconductor elements 103a and 103b are bonded onto a lead frame 101 with a bonding material 102.
The power semiconductor element 103b and the other power semiconductor element 103a or the lead frame 101 are electrically connected to each other via, a bonding wire 104 made of, for example, Al. The semiconductor device 100 further includes a heat-dissipating plate 105 exposed from molding resin 106.
For electrical insulation between the lead frame 101 that carries a current and the heat-dissipating plate 105, an insulating layer 107 is formed between the lead frame 101 and the heat-dissipating plate 105. The insulating layer 107 is an epoxy-resin insulating sheet containing a ceramic filler.
For example, a home power conditioner requires a dielectric voltage of 2.5 KV between the lead frame 101 and the heat-dissipating plate 105. In this case, the thickness of the insulating layer 107 is set at, for example, 0.15 mm to 0.3 mm.
Moreover, the semiconductor device 100 needs to be reduced in size. Thus, the proposed smaller semiconductor device 100 includes the lead frame 101 and the heat-dissipating plate 105 that are substantially identical in size. However, if the lead frame 101 and the heat-dissipating plate 105 have the same size, an air layer 108 (see FIGS. 6(a) and 6(b)) in the molding resin 106 may deteriorate the dielectric voltage.
This point will be described below in accordance with FIGS. 6(a) and 6(b).
FIGS. 6(a) and 6(b) show internal cross-sectional views of the semiconductor device and cross-sectional schematic diagrams of the positional relationship among the lead frame 101, the heat-dissipating plate 105, and the insulating layer 107. In FIG. 6(a), the lead frame 101 and the heat-dissipating plate 105 have the same size. In FIG. 6(b), the heat-dissipating plate 105 is larger than the lead frame 101. FIG. 6(a) shows that the air layer 108 is located near the edge of the heat-dissipating plate 105. The air layer 108 is, for example, a void or the like that is caused by a resin flow in a molding step. It is quite difficult to control the position and size of the void and thus as shown in FIG. 6(a), the edge of the lead frame 101 and the edge of the heat-dissipating plate 105 may be connected to each other via the air layer 108. Since the dielectric voltage of air is about 3 KV/mm, the application of a voltage of 2.5 KV may cause a dielectric breakdown at the position of the air layer 108 so as to pass a current from the lead frame 101 to the heat-dissipating plate 105. The heat-dissipating plate 105 in use is typically attached to an Al heat sink and so on. In the event of a dielectric breakdown, a current may pass through the Al heat sink.
In the semiconductor device 101 of Patent Literature 1, as shown in FIG. 6(b), the edges of the lead frame 101 and the heat-dissipating plate 105 are separated from each other, obtaining insulation reliability with a physical distance even in the presence of the air layer 108.
As shown in FIG. 7, Patent Literature 2 discloses a semiconductor device 201 including a heat-dissipating plate 203 coated with an insulating coating 202 and a connecting terminal 204 on which a device 205 is disposed. The heat-dissipating plate 203 is connected to the connecting terminal 204. In the semiconductor device 201 of Patent Literature 2, the edge of the heat-dissipating plate 203 is coated with the insulating coating 202. Thus, insulation reliability can be obtained even in the presence of the air layer 108.